1. Field of the Invention
This invention relates to transceiving signal antennas, and more particularly to a mobile antenna having a connected network allowing signal transmission over a broad band of frequencies.
2. Description of the Related Art
This invention relates to a certain type of mobile antenna, illustrated in FIG. 1, having: a threaded base mount connector C attached to a car or vehicle body, a housing H that mates to the threaded connector, and an antenna or collinear antenna rod A that fixes to the housing H often via a screw ferrule F or the like.
The base mount connector C allows antennas to be interchanged or replaced on the same common base. Variations on this system are widespread and supported by many manufacturers in the United States and other countries in a generally recognized industry standard.
The housing H usually holds an impedance matching network that, with the dimensions of the antenna A, sets the gain and operating frequency for the antenna system as a single unit. Matching networks include: "L" networks that are used to step the impedance up or down, simple inductors to resonate the capacitance of the antenna rod, or tapped inductors to accomplish both the inductive resonance and an impedance transformation.
The antennas attached to this housing fall into three general categories: antennas that are equal to or slightly shorter than 1/4 wavelength long; antennas that are 1/2 wavelength long, or antiresonant, so they do not require a ground plane; and antennas that are 5/8 wavelength long. Antennas with multiple elements in series, which elements are phased to radiate to the broadside, will include an element in one of these categories to permit impedance matching.
Such antennas have a limited operating bandwidth and are not as useful as they might be. The bandwidth is limited by the small diameter and the electrical length of the antenna rod, and by the requirement for a matching network that uses a reactance to resonate with the antenna rod. The bandwidth is further narrowed when additional collinear elements are added to increase the gain of the antenna. These limitations and their consequences are described in such references as those by L. J. Chu, "Physical Limitations of Omni-Directional Antennas," Journal of Applied Physics, Volume 19, December 1948, pp. 1163-1175; and Harold A. Wheeler, "Fundamental Limitations of Small Antennas," Proceedings of the IRE, December 1947, pp. 1479-1484 and "Wideband Matching Area of a Small Antenna," IEEE Transactions on Antennas and Propagation, March 1983, pp. 364-367. In accordance with Maxwell's Laws relating to electromagnetism, the useful bandwidth of an omni-directional antenna is fixed by the size and gain of the antenna.
Modern radios with their broadband capacity and solid state circuits have operating capabilities far in excess of the limited bandwidth of such antennas. Generally, modern radios are limited by their connected antennas, restricting the efficiency of such radios. FCC bands are usually wider than the bandwidth of an efficient and gainworthy antenna, and when elements are added to an antenna to add desired gain, the antenna's bandwidth is narrowed. Consequently, otherwise available frequencies available for use in an established FCC band are beyond the capacity of modern radios using present antenna systems. Increasing the bandwidth of the associated antenna would allow modern radios to make use of more, if not the entire, available FCC frequency band.
A number of strategies have been developed to broaden the operating bandwidth of these mobile antennas. These strategies are illustrated in FIGS. 2 and 3 and in U.S. Pat. No. 3,950,757 entitled "Broadband Whip Antenna" issued to Blass on Apr. 13, 1976 and U.S. Pat. No. 4,028,704 entitled "Broadband Ferrite Transformer Fed Whip Antenna" issued to Blass on Jun. 7, 1977. The strategy outlined in these patents has the disadvantage of high VSWR (Voltage Standing Wave Ratio). Modern radios often will not tolerate a VSWR in excess of 2:1 at their output terminals and current industry standards steer installers away from such VSWR ratios.
Q Loading: Introducing a resistance R into either the rod or the matching network lowers the "Q" of the antenna system and increases the bandwidth. One popular approach is to replace the whip portion of the antenna by winding a resistive wire on a fiberglass core of small diameter. This is shown in U.S. Pat. No. 4,160,979, "Helical Radio Antennae."
Another commonly encountered approach is to use a resistive wire or a low "Q" capacitor in the matching network. Still another approach is to place a fixed resistor R into the antenna rod at the point of maximum current. This is described by Edward E. Altshuler, "The Traveling-Wave Linear Antenna," IRE Transactions on Antennas and Propagation, July 1961, pp. 324-329. Q Loading reduces the efficiency of an antenna by 50% or more.
Adding Diameter: Increasing the diameter of an antenna at a voltage node N increases its operating bandwidth. This is most easily done with a one-half wavelength (1/2 .lambda.) antenna, which, because it is fed at a voltage node, the diameter of the antenna may be increased in the area of the feed point which places the increased mass close to the fixing point of the antenna assembly. Adding diameter in this fashion only marginally increases the bandwidth of an antenna.
Reactance Compensating Networks: The reactance change with frequency of an antenna network may be nearly cancelled over a band of frequencies by an appropriate compensating network I often using a parallel resonant network to compensate a series resonant antenna and a series resonant network to compensate a parallel resonant antenna.
The technique, including formulas and table for the development of such networks is described in Microwave Filters, Impedance-Matching Networks, and Coupling Structures, by George Matthaei et al., Artech house, Needham, Mass., 1980.
As described by Hugo Pues, U.S. Pat. No. 4,445,122, issued Apr. 24, 1984 entitled "Broad-Band Microstrip Antenna," the compensating network performs best if it is shielded from the associated antenna structure. This reduces coupling between the compensating network and the radiating field generated by the antenna. The current practice has been to place the network inside the automobile body (generally made of conducting metal), and further inside a metal shielding box. FIG. 3 shows such a box B1 adjacent the connector C where one manufacturer places the network in a box on the vehicle side of the base connector.
Another manufacturer places the network B2 in the coaxial cable a distance from the base connector C. This location, as described on page 43-28 of Antenna Engineering Handbook, 3d edition, edited by Richard C. Johnson, McGraw-Hill, Inc., is less than ideal for the requirements involved.
These approaches demonstrate the difficulty of locating the compensating network with the matching network inside the mounting housing. As a result they lack the interchangeable feature otherwise built into a connector-housing-antenna system. The advantage would be regained if the bandpass widening network were placed inside the mounting housing with the antenna matching network.
The difficulties in putting a bandpass filter into the coil housing derive from the following requirements and circumstances:
that the antenna be mismatched at its frequency of lowest VSWR because the available bandwidth increases as the mismatch is increased; PA1 that the tuning of the network takes place when the antenna is attached because the reactive elements of the antenna matching network are partially shared with the bandwidth-expanding network; PA1 the reactive elements of the bandwidth-widening, or compensating, network must be tuned to the same frequency and must be shielded from each other and from the antenna while simultaneously compensating for any effect of coupling to the shielding structure; PA1 that the resonant networks have parasitic impedances which transform the coupled resistances in ways that cannot be accurately modelled on a computer; PA1 that the network geometry be suitable for a wide variety of rod impedances; and PA1 that the impedance break of the connecter interface must be compensated by the bandwidth-widening network. PA1 1) A housing holding the bandwidth-compensating network that is constructed with a metal top cap and metal bottom ring. The cap and ring shield the inductors from the antenna field and are insulated from each other by a plastic cylinder or other insulation. PA1 2) An antenna and matching network, affixed to the housing, having: PA1 3) A compensating network, consisting of: PA1 4b) vice-versa, i.e., the antenna, shield, and inductor are tuned for zero reactance at an approximate frequency one-half to one percent (1/2-1%) higher than the center frequency and the compensating network is separately tuned to the center of the desired bandwidth.